EE201
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Title: Measurement of an Unknown Resistance by
1 .
Current-Voltage Method
2 .
Wheatstone Bridge method
Aim: To Measure the value of an unknown
resistance and to determine the characteristics of a resistor.
FIRST METHOD
(Current-Voltage
Method)
Apparatus:
1 . Unknown Resistor
2 . D.C Supply (0-15V, 2A)
3 . D.C Voltmeter (0-30V)
4 . D.C Ammeter (0-2A)
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THEORY:
The theory required for this experiment was an understanding of Ohm’s Law. Ohm’s Law is the algebraic relationship between voltage and current for a resistor. Resistance is the capacity of materials to impede the flow of current or electric charge. Ohm’s Law expresses the voltage as a function of the current. It was also necessary that the concept of measurement accuracy be understood. This is discussed below.
Accuracy is of primary importance in an experimental work. The tolerance
quoted by the meter manufacturer allows us to calculate the accuracy of any reading
taken with that particular meter. For example, assume that the dc voltage scale on a particular multimeter is rated at ± 3% of full scale. This means that a reading on the 10V scale is accurate to (± 0.03%)(10) = ± 0.3V. Thus, a reading of 9V on
the10V scale indicates a true voltage, which lies between 8.7 and 9.3 V. A reading of 1V on the scale would indicate a true voltage between 0.7 and 1.3 V. At this point, the error
is ± 30%! Any reading less than 10% of full scale should be viewed with suspicion
since most meters are very inaccurate n this range
APPLICATION OF OHM'S LAW
By using Ohm's law, you are able to find the resistance of a circuit, knowing only the voltage and the current in the circuit.
In any equation, if all the variables (parameters) are known except one, that unknown can be found. For example, using Ohm's law, if current (I) and voltage (E) are known, resistance (R) the only parameter not known, can be determined:
Basic formula
Remove the divisor by multiplying both sides by R
Result of step 2: R x I = E
To get R alone (on one side of the equation) divide both sides by I
The basic formula, transposed for R, is
For
Procedure and Circuit Diagram, See your Electrical Laboratory Manual on page 3.
Fill Up
Tables in Your Manual and Continue To….
METHOD 2
(Wheatstone
bridge Method)
Apparatus:
1. Unknown Resistor
2. D.C Supply
3. Wheatstone bridge
For
Procedure and Circuit Diagram, See your Electrical Laboratory Manual on page 5
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THEORY:
A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. Its operation is similar to the original potentiometer. It was invented by Samuel Hunter Christie in 1833 and improved and popularized by Sir Charles Wheatstone in 1843. One of the Wheatstone bridge's initial uses was for the purpose of soils analysis and comparison.
For measuring accurately any electrical resistance Wheatstone bridge is widely used. There are two known resistors, one variable resistor and one unknown resistor connected in bridge form as shown below. By adjusting the variable resistor the current through the Galvanometer is made zero. When the current through the galvanometer becomes zero, the ratio of two known resistors is exactly equal to the ratio of adjusted value of variable resistance and the value of unknown resistance. In this way the value of unknown electrical resistance can easily be measured by using a Wheatstone Bridge.
Working
The three known resistances of the parallel branches are already known. The current is allowed to pass through the circuit. When the current passes through the galvanometer, the three resistances are adjusted is such a manner that the galvanometer reading shows zero. The same process can also be carried out by varying the resistance of only one the resistor. Let’s see how.
Now
suppose there are 4 resistors R1, R2, R3 and Ru. Ru is the resistor whose
resistance is to be found and R2 is the only adjustable resistor. The
arrangement is as shown in the figure. R1 and R2 are on one leg and R3 and Rx
are on the other leg. Now if is the ratio of resistances of known path, R2/R1
is equal to the unknown path Ru/R3, then the reading at the galvanometer
located at the center will show zero. This is done by varying the resistance of
R2.
At
the point the current in galvanometer is zero; the resistances of all the three
known resistors are noted. The resistance of the fourth unknown resistance can
be found out by the formula
R2/R1
= Ru/R3
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Applications
Wheatstone
bridge is widely used for measuring small resistances and therefore it is used
in applications such as strain gauges and resistance thermometer. Mostly a part
of electrical measurement circuits, wheatstone bridge is an integral part of
low temperature alarms. For e.g., a thermistor’s resistance is measured by
placing the thermistor in place of the unknown resistance in the method
described above. The resistance of the thermistor changes as the temperature it
is exposed to changes. The temperature and resistance of the thermistor are
inversely proportional to each other. This means that if the temperature of the
thermistor increases, its resistance decreases. The difference in temperature
between the two legs is reflected across the bridge to which an alarm is
attached. The temperature difference activates the alarm.
PRECAUTIONS:
For
Precautions, See General Electrical Lab
Precautions
Answers
to Questions:
No Questions in this experiment.
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